‘Time-travel’ technique tackles force-feedback delays

Internet lag can make collaborating in a virtual world with force-feedback difficult

(Image: IEEE)

Delays on the internet are distracting enough when you are trying to watch a video. But imagine how frustrating they could be during trans-Atlantic fencing matches with force-feedback swords.

Synchronising so-called haptic (touch-based) devices and virtual environments over the internet is difficult because large amounts of information must be sent every few seconds. Delays may also last only last a few hundred milliseconds or so, but over time they add up to introduce serious discrepancies at either end. This can be a major problem if, for example, a force-feedback device is being used to remotely-operate a surgical instrument.

Now researchers in the UK and US have come up with a technique to tackle the synchrony issue. Roger Hubbold and colleagues from the University of Manchester, along with a team from the University of North Carolina at Chapel Hill in the US, developed a system that employs a clever “time-travelling” technique to make interaction tasks more seamless.

To test the system, they used a virtual world in which two users, on either side of the Atlantic, must to cooperate using haptic interfaces to fit a virtual fuel control box onto an aircraft engine (see image).

“Rollback” method

Their system employs peer-to-peer networking, meaning it communicates without the need for a central server. This is faster since the software used by each haptic device does not have to wait for a response from the server when a movement is made.

Crucially, it also uses a technique dubbed “rollback” to deal with communication delays. When a delayed message is received, the software calculates how things would have turned out if it had arrived on time. It then alters the virtual world as subtly as possible so that these effects are expressed in the “true”, real-time situation.

For example, a virtual object will automatically move fractionally faster to compensate for its previous lag in movement. “When [a messge] arrives, you go back in time to see what effect it would have had,” explains Hubbold.

Fifteen volunteers tried the system with and without the rollback technique. Without it, the virtual worlds experienced by each user rapidly became unsynchronised. After 2 minutes, objects had diverged on the two screens by up to 25 centimetres and misaligned by up to 50 degrees. With the rollback mechanism switched on, divergences were only 2 centimetres and a few degrees at most. Users very rarely noticed the continual adjustments.

Game playing

Hubbold says a reliable way to avoid lag will be needed if more sophisticated virtual worlds and haptic systems are to take off. Current computer games can usually get away with ignoring the problem, because they do not involve very detailed physics simulations. “As games become more advanced they would have to start considering it,” he told New Scientist.

Anthony Steed, who researches virtual environments and visualisation at University College London, UK, agrees&colon; “Latency, or lag, is probably one of the biggest system-building challenges that exists” in these areas.

The research was presented at the IEEE Virtual Reality conference in March 2007.